The European Telecommunications Standard Institute-Digital Mobile Radio (ETSI-DMR) standard (ETSI TS 102 361-1) describes a TDMA air interface protocol. Before a subscriber unit is allowed to receive or transmit on a TDMA channel, it must ensure that it is synchronized with the desired timeslot. To that end, the ETSI-DMR standard provides a TDMA Channel (TC) bit which informs the receiving device whether the next timeslot to be received is timeslot 1 or timeslot 2. The TC bit, along with other protocol bits that are of no particular importance relative to this disclosure, is protected with forward error correction (FEC) parity bits (e.g., using a Hamming (7,4) code) to improve the probability that it is received correctly in the presence of unavoidable channel impairments; the protocol, however, does not provide any error detection parity bits (e.g., cyclic redundancy check or checksum) which may enable the receiving device to determine whether the TC bit has been received correctly. Consequently, inspecting a single TC bit provides the receiving device an indication of which timeslot follows, but with limited confidence that the timeslot has been identified correctly.
Inspecting multiple adjacent TC bits and noting whether they alternately indicate timeslot 1 and timeslot 2 can provide the receiving device insight as to the correct, incorrect, or uncertain identification of the timeslots, but inspecting multiple TC bits requires additional time because they are only provided on the channel periodically (e.g., every 30 ms). A receiving device may need to receive 4 to 8 adjacent TC bits (correspondingly, 120 to 240 ms after synchronizing with the frequency) to identify a timeslot as being timeslot 1 or timeslot 2 with high confidence, and possibly need to receive more TC bits if errors in the alternating timeslot 1-timeslot 2 pattern are noted. This amount of time can be restrictive and limit performance in systems that require the receiving device to change channels frequently. An example of such a system is one that requires the receiving device to search for call activity of interest by sequentially stepping through, or scanning, a list of channels.
According to the ETSI-DMR standard, the TC bits are transmitted in the Common Announcement Channel (CACH). The CACH is transmitted by a repeater, and is positioned between the transmission of timeslot 1 and timeslot 2. The ETSI-DMR standard also provides for direct mode (or talk-around) transmissions, where subscriber units may communicate without the facilitation of the repeater; the ETSI-DMR standard, however, only allows up to one subscriber unit to transmit in direct mode on a frequency at a time, which leaves a significant portion of the channel unoccupied. Since there is no repeater in direct mode transmissions, there is no CACH message to identify timeslots on the channel. The transmitting subscriber unit cannot provide the CACH information, because a 2.5 ms “guard time” between the two timeslots must be reserved to ensure two transmitting subscriber units do not interfere with one another due to factors such as propagation delays and drift of the reference oscillator.
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